Observed peak flux, $I_{1.3\mathrm{m}\mathrm{m}}^{\text{obs,peak}}$, versus dual-band spectral index, α. The black circles with error bars show the dual-band detections, of which the KDE is shown in gray contours. The self-consistent dust model with micron-sized (a_max = 0.01 mm) dust grains (Liu 2019) are shown with shaded regions that cover a temperature range of 6–48 K. The blue, pink, and gray colors represent beam diluted class 0/I YSOs with FWHM sizes of 50, 100, and 200 au, respectively.